1. Field of the Invention
The present invention is concerned with elastomeric materials and methods of making them, including meltblowing methods for making elastomeric fiber nonwoven mats, and is more particularly concerned with such materials made at high temperatures from an extrudable composition comprising a particular class of styrenic elastomeric rubbers.
2. Description of the Related Art
Meltblowing techniques for forming from thermoplastic resins very small diameter fibers, sometimes referred to as microfibers or meltblown fibers, are well-known in the art. For example, the production of fibers by meltblowing is described in an article entitled "Superfine Thermoplastic Fibers", appearing in Industrial and Engineering Chemistry, Vol. 48, No. 8, pp. 1342-1346. This article describes work done at the Naval Research Laboratories in Washington, D.C. Another publication dealing with meltblowing is Naval Research Laboratory Report 111437, dated Apr. 15, 1954. Essentially, the meltblowing technique comprises heating a thermoplastic fiber-forming resin to a molten state and extruding it through a plurality of fine orifices into a high velocity heated gas (air) stream which attenuates the threads of molten resin being extruded through the fine orifices to form meltblown fibers of a diameter less than the diameter of the orifices. U.S. Pat. No. 3,849,241 discloses the manufacture of nonwoven mats by meltblowing and describes, starting at column 4, line 57, meltblowing fibers by extruding degraded fiber-forming thermoplastic polymer resins having diameters of from about 0.5 to about 400 microns. The patent discloses that the diameter of the attenuated fibers will decrease as the gas flow rate (through the gas outlets which are located on either side of the orifices) increases. The patent further notes that at low to moderate rates of gas flow the extruded fibers are essentially continuous with little or no fiber breaks and that fibers produced in such low to moderate gas flow rate regimes have diameters of, preferably, from about 8 to 50 microns. This patent also discloses that the fiber-forming thermoplastic polymers are subjected to controlled thermal and oxidative degradation at temperatures of from about 550.degree. F. to 900.degree. F. (288.degree. C. to 482.degree. C.), preferably from about 600.degree. F. to 750.degree. F. (316.degree. C. to 399.degree. C.) for a period of time to cause the requisite extent of resin degradation. Typical fiber-forming thermoplastic resins are listed at column 4, line 35 et seq and commercially useful resin throughout rates are stated to be from about 0.07 to 5 grams per minute per nozzle orifice, preferably at least 1 gram per minute per nozzle orifice. Degradation of such resins is necessary in order to reduce their viscosity sufficiently for extrusion and attenuation by the high velocity gas stream.
Of course, there is a limit to the degree of degradation which can be imposed on a given resin without unduly adversely affecting the desired properties of the product obtained therefrom. This is particularly so in the case of elastomeric fiber-forming resins such as styrenic ethylene-butylene block copolymers where degradation of the tri-block copolymer can result in the formation of a di-block material which is non-elastic. For example, Technical Bulletin SC: 38-82 (October, 1982) and SC: 39-85 (January, 1985) of Shell Chemical Company, Houston, Tex. in describing the styrenic ethylene-butylene rubbers sold by it under the trademark KRATON, respectively state with respect to the materials designated as G 1650 and G 1652, that compounding temperatures of the rubber should not be allowed to exceed 525.degree. F. (274.degree. C.) and that a fire watch should be maintained if 475.degree. F. (246.degree. C.) is reached. With respect to the KRATON rubber designated GX 1657, Technical Bulletin SC: 607-84 (September, 1984) of Shell Chemical Company gives a warning not to allow the resin temperature to exceed 450.degree. F. (232.degree. C.) and to maintain a fire watch should that temperature be reached. Material Safety Data Sheets 2,136 (Revised 1-83) and 2,031-1 (Revised 1-83) of Shell Chemical Company state respectively for the GX 1657 and G 1652 materials that processing temperature of the resin should not be allowed to exceed 550.degree. F. (287.8.degree. C.) and a fire watch should be maintained if that temperature is reached. Shell Chemical Company Technical Bulletins SC: 65-75 "KRATON Thermoplastic Rubber" and SC: 72-85 (March, 1985) "Solution Behavior Of KRATON G Thermoplastic" are among numerous publications of Shell Chemical Company which give detailed information concerning various grades of KRATON thermoplastic rubbers. The KRATON thermoplastic rubbers are A-B-A block copolymers in which the A endblocks are polystyrene and the B midblock is either poly(ethylene-butylene) (KRATON G grades, sometimes referred to as "S-EB-S" resins) or polyisoprene or polybutadiene (KRATON D grades).
Shell Chemical Company Technical Bulletin SC: 198-83, (Revised 7-83) at page 19, gives examples of commercially available resins and plasticizers useable with KRATON rubber formulations, distinguishing between rubber phase (B midblock)-associating materials and polystyrene phase (A endblock)-associating materials.
U.S. Pat. No. 4,323,534 (des Marias) discloses a process for meltblowing a blend of an A-B-A block copolymer wherein B is poly (ethylene-butylene) and A may be, for example, polystyrene or poly(alpha methylstyrene) with from about 20 percent to 50 percent, by weight, of a fatty chemical such as stearic acid. KRATON materials, as described above, are exemplified as the A-B-A block copolymer material. An extrusion temperature range of up to 240.degree. C. (column 8, line 64 et seq) is disclosed for the meltblowing operation, which temperature range is generally within that recommended by the above-mentioned Shell Chemical Company technical bulletins. In order to improve the physical properties of the meltblown material, substantially all the fatty chemicals are leached out of the nonwoven mat of extruded microfibers by soaking the mat in alcohols having good solubility for the fatty chemical utilized.
U.S. Pat. No. 4,355,425 (Jones) discloses an undergarment which may be made of a fiber formed by meltblowing a blend of a KRATON G rubber with a fatty chemical such as stearic acid. The examples are apparently limited to KRATON G 1652 block copolymer. An extrudable composition stated to be particularly useful for the purpose (column 4, line 24 et seq) is a blend of KRATON G 1652 rubber and 20 percent by weight stearic acid as well as other ingredients. The disclosed extrusion temperature of 390 degrees Farhrenheit (199.degree. C.) for the composition (column 5, lines 14 and 19) is within the temperature range set forth in the above-mentioned Shell Chemical Company technical bulletins. It is further stated that fibers for making the material can be meltblown as taught in U.S. Pat. No. 3,825,380, which discloses a die configuration said to be suited for meltblowing fibers. It should also be noted that the procedures of Jones, as was the case with the procedures of des Marais, indicate the desirability of leaching out the fatty chemical after formation of a fibrous nonwoven web or film from the blend of fatty chemical and KRATON G. See, for example, column 5, lines 60 et seq.